Heat exchanger

ABSTRACT

Element for heat exchanging between fluids, where the flowing canals for the fluids are formed of slits on either side of a thin, folded sheet material ( 1 ), and that the ratio between the slit width ( 24 ) of the canals and depths ( 25 ) in the slits is less than 0.15 times the thickness of the sheet material.

The present invention relates to a heat exchanger element for exchangingof heat between two media, an application of the element.

Heat exchangers are well known and used in many connections. The presenttypes of heat exchangers use pipes or sheets as dividing walls betweenthe two media. Based upon considerations as to heat transfer, weight anduse of materials, it is an object to develop heat exchangers in whichthe dividing walls are as thin as possible, but where the structuralconstruction of the exchanger elements still has the necessary strengthto be able to withstand the actual pressures in various applicationperiods.

Pipe exchangers are designed with several pipes in the common mantle, oras two coaxial pipes. Such exchangers are well suited to withstand highpressures, and they are also relatively well secured against leakagebetween the two media. However, since pipes are much more expensive thanthe corresponding area as sheets, are production costs relative high.

On the other hand are sheet exchangers specially used in applicationswhere there are no absolute requirements as to tightness, and where theexchanger's capacity is high.

In both types of known heat exchangers, are the two media's flowprofiles far from ideal. For the known sheet exchanger, which makes abasis for comparison for the present invention, the problem is tied to alimited flow velocity between the sheets. The limitation lies in thefact that input and output channels in each corner are tight in accountof the geometry, and the media must make a sharp 90° turn with resultingfall in pressure. It therefore is this geometrical matters and not theflow velocity in the exchanger which provides the limitation for anacceptable pressure fall.

The flow velocity along the exchange element is decisive for the heattransfer between the medium and the area's α-value, and is applied inturbulent flowing of water in calculation of the α-value as a factor offor example 1 at 1 m/sec and 1.8 at 2 m/s.

Previous heat exchangers are known, where each sheet is folded in zigzagform such as it on each side is formed channels through which the actualmedia circulate, see for example U.S. Pat. No. 1,601,637, EP 0 197 169and SE 362 137. Common for all these is that they have channel profilesthat do not combine necessary area density with a structural strength topermit the use of thin sheets, and such narrow channels that the flowvelocity will be high enough. This is relatively fatal because the lowheat transfer will demand a larger exchanger, which will mean stilllower flow velocity.

Principles for distillation at recompression of steam are known. FIG. 7shows for example a chamber 18 which is divided with a dividing wall 19,in which there is a liquid 20 which is heated to the boiling point. Thesteam 21 from the liquid are sucked into a fan or compressor 22 and arepressed from this one into a room under the dividing wall 19. Because ofa higher pressure, the steam will here condense at a temperature whichis higher than the temperature of the vaporizing period.

The heat of condensation will therefore go over to the liquid andevaporate a corresponding quantity of new liquid. The condensate 23 canthen be tapped at the same time as new liquid is led to the evaporationside.

This principle is applied to day industrially till drying of effluentsas for example at production of dry milk, cellulose etc.

The effectiveness factor of the process is calculated from the energy ofevaporation respectively fan energy. For a given quantity of distillate,is the fan energy determined by the pressure ratio between evaporationand condensation.

The pressure ratio and therefore the energy consumption depend on howeasy necessary heat transfer between the division wall takes place.

Finally, this may be expressed such as the dividing wall's heattransfer, k-value is a decisive criterion as to the size of the areawhich is necessary on the dividing wall.

With water vapour, is a limit in pressure ratio for one step fans at1,15, which means about 3,5° C. higher temperature of condensation. Withan evaporation temperature of 100° C., the condensation will take placeat 103,5° C.

Except for parameters like dust and such, the total heat transfer,namely the k-value, will be determined by:k=1/(1/α1+s/λ+1/α2)where α1=heat transfer on the condensation side, α2=heat transfer on theevaporation side, s=sheet thickness and λ is equal to the sheetmaterial's heat transfer coefficient period.

The lowest value of one of the α-values will then give an asymptoticborder value for how high the k-value may be.

With a vertical surface one can achieve a relatively high α1-value forcondensation. The value will be higher the less height the surface has.With a height of 50 mm, the α-value is of approx. 13 500 W/m²° C.

However, it is the α2-value on the evaporation side which definitely isthe limiting factor at such low temperature differences which can beachieved here. The case is that the temperature difference between thesurface and the boiling point minimum must be 7° C. before the waterwill boil with bubble from the heating surface. With this temperaturedifference, the water evaporates by the fact that it forms small steambubbles in the water itself. This form for boiling is called convectionboiling because the heat transfer takes place by convection period.

With convection boiling with a temperature difference of 3,5° C., onecan not obtain a higher α2-value than about 1 800 W/m²° C. The k-valuewill therefore be so low that distillation of water at the abovepressure and temperature will require 8,8 m²/kg distillate per minute.This is too high to obtain realistic dimensions and costs for suchdistillation plant.

The only known method to increase the heat transfer at convectionboiling, is to set the water in motion with respect to the heatingsurface, for example by means of a stirring device, propeller or pump.

This is achieved with the heat exchanger according to the presentinvention as it is defined with the features set forth in the claims.

For the heat exchanger according to the invention, one of the goals hasbeen to achieve a higher heat transfer by self circulation withoutmechanical aids. The density of the small steam bubbles which is formedin the water at convection boiling, are, however, at the beginning toosmall to achieve any positive degree of self circulation. The density ofthe steam bubbles may be increased by increasing the heat radiationagainst the water. This may, with the given parameters, only be achievedby surrounding small quantities of water with a large heat supplysurface, in practice by confining the water in a narrow slit between thesheets.

FIG. 1 of the drawings shows a perspective view of a heat exchangerelement according to the invention,

FIG. 2 shows the exchanger element on FIG. 1 placed in a exchangerhousing,

FIG. 3 shows a view of the invention used in an air/liquid heatexchanger,

FIG. 4 shows a section of the heat exchanger of FIG. 3,

FIG. 5 shows schematically the first stage for production of the heatexchanger element according to the invention,

FIG. 6 shows the second stage for production,

FIG. 7 shows schematically the principle for the process, and

FIG. 8 shows schematically a heat exchanger element with plane canalsand element walls.

The principle for the heat exchanger is shown in FIG. 8. The sheet ishere folded together into slits with the slit width 24 and the slitheight or depth 25, where the water appears in the slits that are opento the top, and the condensing steam appears in the slits that are opentowards the underside.

It is apparent, that when the slit width 24 gets narrower, will theactual heat flux q (W/m2) radiate a decreasing amount of heat, and adensity of the steam bubbles increases. At a certain border the steambubbles will start to combine to bigger bubbles. This will set the waterin strong motion, the heat flux will increase and the quantity of steambubbles increases, meaning that a self amplifying reaction occurs. Thequantity of steam will then be so large that it in the major part of thecross section forms a two faced stream with the steam in the middle ofthe slit and a thin film of water which is pulled along along thesurfaces.

Trials have shown that, with water at 100° C. and a slit with of 1,5-2mm, is this border at a temperature difference of only 1,8-2° C., andthe α-value is increased from about 1 800 W/m²° C. to 18 500 W/m²° C. onthe evaporation side. This is about at a level that can be achieved byordinary fast boiling with a temperature difference of about 18° C. orwith film evaporation, which is often used in the above mentionedindustrial drying plants.

It has been found that optimal results are achieved when the slit width24 of the slits forming the fluid flow canals 26 on either side of thethin, folded sheet material 1 and the slit height or depth 25 in theslits forming the flow canals 26 are in a certain ratio to each otherand to the sheet thickness. Thus the ratio between the slit width 24 andthe slit depth 25 should be less than 0.15 times the thickness of thesheet material. The height of the folds must be limited both to avoidgetting too high counter pressure in the steam and so that thecondensate on the underside should have a short runoff distance period.A slit height h=50 mm seems to give an optimal result for both sides.

The heat transfer on both sides becomes so good that the sheet thicknessstarts to have a negative effect on the total k-value. For this reason,but also on account of the weight and the costs, it is an advantage tohave a thin sheet as possible. It is now often used 0.4-0.5 mm titaniumsheets in the heat exchangers.

With as narrow slits as are necessary here, straight slits as shown onthe principle figure could easily be pressed together at a relativelysmall pressure difference.

In the heat exchanger according to the invention, one has arrived at amethod for construction of the element, corrugating of the sheet, whichprevents pressing together, but at the same time does not hinder accessfor cleaning or closes for the flow through of the media, as shownschematically on FIG. 1.

To isolate the two media sides from each other, the slits in theexchanger element must be closed at the ends. In many applications thiscan easily be done by moulding the ends together in a suitable material.

The ends may also be closed by welding the slits together. Anothersolution is to press a lid of for example rubber against the ends.

With its high compactness and direct access from both sides forcleaning, is the heat exchanger well suited as a general heat exchanger,for example as a motor cooler. Also in this case will the narrow slitsgive a special thermal effect. A through flow of liquid will result inthat the heat transfer increases with the flow velocity period. When theslit is narrow, the turbulence near the surface is higher than with abroad slit or a pipe, even if the mean flow velocity is the same.

The exchanger element is in this application capsulated into mantelhalves equipped with respective input and output at the ends.

The problem may be exemplified with a motor cooler where the temperatureis to be lowered about 5 to 8° C. With a canal height of 50 mm, thecanal width must not be more than 1.5 to 2 mm wide in order to achieve avelocity of 1.5 to 1.8 m/s. This requires a minimal pressure differencefor a thin sheet to buckle, and thereby reduce or close the canals onthe opposite side.

The element consists of a thin sheet which is folded to a bellows whereall the folding plans are curved in the same direction. Pressure in thecanals will affect the canal walls' form if the pressure in theneighbouring canals is different. One side in the canal tries tostraighten out while the other side pulls the profile inwards such thatthe forces approximately equalise each other. An exchanger element 1 ismounted between a top mantel 2 and a bottom mantel 3 which both haveinput and output canals 4 and 5 in each end. The mantels 2, 3 may alsoexercise a pressure against the element's canal tops, such that theelements curves are biased and thereby get a further increase structuralstrength. The curves in the canals' wall surfaces are preferablyparallel.

Prototype tests have been carried out with a motor cooler with canaldimensioning as mentioned in the above example, and with a titaniumplate only 0.5 mm thick. The construction was able to withstanddifferential pressure of more than 5 bars without any sign ofdeformation of the canal walls. High area density compared to the flowthrough area proved to give α-value of more than 30% higher than for apipe with the same flow through velocity period. The total heattransfer, k-value, is essentially higher for conventional sheet heatexchangers.

In addition to heat exchanging between liquids, the heat exchangerelement according to the invention will be suited for heat exchangingbetween a liquid by convection, evaporation or condensation on one side,and the gas, for example air, on the other side.

Such an embodiment is shown on FIGS. 3 and 4. On the liquid side it isfastened a sheet 6 which covers the element and is fastened to the canaltops. For very high pressure differences, for example in cooling plants,it could be actual to use wart-like corrugation to increase the activesurface.

The invention has some of its foundation in the need for an effectiveand reasonable combined condenser and evaporator for distillation at therecompression of steam period. One such distillation apparatus is inreality a heat pump where steam from the evaporation side is sucked intoa fan and pressed by this one into the condenser side where it onaccount of the increased pressure condensers at a temperature which ishigher than the evaporation temperature. Thereby the condensation heatis transferred to the evaporation side such that a corresponding newamount of liquid will be evaporated. On this way the heat energy isrecirculated internally.

On for example ships are used distillation apparatus, so calledevaporators, to produce fresh water from sea water. These use excessheat from the motors as the source of energy in a special boiler, andhave a special condenser which is cooled with sea water.

The evaporation of sea water has, however, great problems with thearrival at solutions which give a satisfactory heat transfer at the sametime as they keep clean. Especially large problem is the so called“scaling”, that is that salt and calcium precipitate and forms a hardand heat insulating cover on the heat exchanger surfaces.

The problem is especially caused by the fact that when a steam bubble isformed on a surface of a boiler element, it leaves salt and drymaterials that the water contained as crystals on the surface. Theseresiduents will soon form a cover which is fastened by heat because thetemperature below the cover rises as result of the increase isolationeffect period. Scaling is reduced by application and boiling at lowtemperature, that is low pressure and dosing with chemicals.

It is correct as it is believed to day that problem with scaling isincreased at increasing temperature, but it is not correctly assumedthat the temperature itself is the fundamental course. This is in thedifference between the boiler element's surface temperature and theboiling point.

A study of the different phases with boiling of water at atmosphericpressure was clearly that the boiling happens at the convectionevaporation at a temperature difference between 1 and 7 K, from 7 to 26K, with bubble evaporation at the surface of the element and more than26 K at film evaporation. In the different phases the heat transferα-value varies strongly.

The temperature difference at boiling in conventional evaporators liesin the area 15-20K, and the boiling therefore takes place in the bubblephase period. The steam bubbles are formed on the boiler element'ssurface, and this is the main cause of the scaling problem.

At boiling in the distillation apparatus with the recompressing of thesteam, the temperature difference will, because of the power consumptionof the fan, be less than 1.5 K, that means that the boiling happens byconvection, and the steam bubbles are formed in the water and not on theheat exchanger's surface. Salts and dry materials will then follow thesteam bubbles up to the surface and follow the excess water out. To holdthe concentration down, it is, as in conventional evaporators, appliedabout twice as much feed water as the produced amount of distillate.

With a temperature difference of 1.5 K, vary low α-value is achieved forthe heat transfer, about 1.5-1,8 kW/m²K. This is so low that it inpractice cannot be applied because the heat exchanger surface will haveto be enormous. It has been shown that the α-value can be increasedsubstantially by using forced circulation, for example a stirringdevice.

The heat exchanger according to the present invention distinguishesitself by a very simple construction, low use of material, very goodheat transfer values and grate usefulness within a number of areas.

In the heat exchanger according to the invention, the canals willcontain a modest demand of water compared to the large surface whichsurrounds the water. Even if the specific heating load W/m² is very low,it will be produced relatively large amount of steam compared to theamount of water in the canals. This creates turbulence in improvedα-value. The turbulence will move the boarder for convection evaporationto a somewhat higher level.

A heat exchanger of the same type and embodiment as the above mentionedproto type has been tested as a combined evaporator and condenser fordistillation by recompressing steam, and it showed that at a boilingtemperature for sea water of 100° C. and a condensation temperature of103° C., it occurs a self amplifying process where α-value suddenlystarts to rise, the turbulence rises further and the heat transferincreases. The self amplifying effect is apparent from the fact that ittakes a few seconds from the first small bubbles appear, until the wateris fast boiling.

In the tests with a distillation apparatus with the heat exchangeraccording to the invention, it shows that the α-value stabilizes at 18.5kW/m²K. The stabilizing indicates the asymptote k-value approachesbecause the heat transfer on the condensation side is hold relativelyconstant.

Horizontal placement has proved to give the best result, but it has alsobeen made tests with the angular placement up to a vertical positionwith relatively acceptable results. At these tests one has covered theevaporation side partly with a sheet 41 with openings at both ends. Thelower opening has been standing under a water column Hwc such that themost of the vapour has been driven through the canals upwards andbrought with it water, such as a substantial recirculation occurs.

It has been shown that the powerful turbulence keeps the evaporationside clean, and no scaling has been observed over a period of time.

At the above mention temperatures, the steam pressure from the fancorresponds to about 1.5 mVs (mVs is the Norwegian definifion of “metresof WC (water column)”), and it is naturally also with this applicationnecessary with the structural strength given by the curves in thecanals. On account of the aggressiveness for the boiling sea water istitan the only useful material, but it is expensive and conducts heatpoorly. This is also important moments for making the sheet as thin aspossible, and the mechanical strength must be taken care of by thestructural construction.

Since the exchanger element according to the invention is formed by along sheet which is folded, the element is well suited for continualautomatically controlled machine production.

For this it has been developed a special method that makes suchproduction possible, where the sheet passes through two machinestations.

In the station in FIG. 5 the sheet is first sharply bent to a step formwith two 90° bends, by a tool knife 7. The tool knife 8 and the toolknife 9 descend simultaneously, and form the sheet between the previousbend, either by forming this in curves as shown in FIGS. 1 and 2, orcorrugating this on a different way, for example parallel, as shown inFIG. 8. The knives 8 and 9 at the same times make the bending anglessharper. All the tool knives 7, 8 and 9 press down against thestationary forming block 10 with the corresponding profiles. A feedingmechanism 11 pulls the sheet forward after the tool knives have returnedto the upper position.

FIG. 6 shows a tool which curves the sheet as shown in FIGS. 1 and 2.With straight corrugated form are the tool knives 8 and 9 and theforming block correspondingly formed with crossing grooves correspondingto the sheet's form.

The pre bent sheet continuous then in the station as shown on FIG. 6.Here each bend is pressed between two pressure plates 12 and 13 on oneside and 14 and 15 on the other side, where the sheet bends get theirfinal width. After the pressure plates 12 and 14 have returned to therear position, the pressure plates 13 and 14 go up and downrespectively, and permit feeding to the next bend. To assure the forwardfeeding, the feeding plates 16 and 15 simultaneously go down and uprespectively.

On FIG. 6 the lower pressure plate 14 is in the forward position, andthe upper pressure plate 12 in the rear position. When even very thinsheets are to be bent as close as indicated here, it is required at theend very large forces that increase exponentially with reducing bendingradius. With the special hinging of the pressure plates 12 and 14, it isachieved a pressing force which is inversely proportional with tangentto the pressure plate's angel. Thereby the force approaches an infinityas the angel and motion approaches zero.

The stations are preferably placed in line and work synchronously.

1. Element for heat exchanging between fluids, the element having fluidsflow canals consisting of slits on either side of a thin, folded sheetmaterial (1) and that a ratio between the canals' slit width (24) anddepth (25) in the slits is less than 0.15 times a thickness of the sheetmaterial, wherein said thickness of the sheet material is about 0.4-0.5mm and said depth of said slits is about 50 mm.
 2. Element according toclaim 1, wherein the element's walls in section are formed as equallycurved surfaces.
 3. Element according to claim 1, wherein the element'swalls are formed as plane surfaces.
 4. Element according to claim 1,wherein the element's walls are produced in titanium.